Arrangement for generating ultrasonic oscillations

ABSTRACT

A control circuit for a transducer comprising a tunable oscillator for driving the transducer at resonance, and a feedback loop responsive to the transducer comprising a phase detector which develops a series of pulse width modulated binary pulses, a D.C. source, an integrator, and various switching means to control the tunable oscillator.

United States Patent Bourgeois et al.

[ June 25, 1974 ARRANGEMENT FOR GENERATING ULTRASONIC OSCILLATIONS Inventors: Ivan Marie Gaston Prudent Bourgeois; Henricus Petrus Cornelis Daniels; Ronny Julius Camiel Cornelius Verlet, all of Emmasingel, Eindhoven, Netherlands Assignee: U.S. Philips Corporation, New

York, NY.

Filed. Jan. 2, 1973 Appl. No.: 319,982

Foreign Application Priority Data Jan. 3, l972 Netherlands ..7200003 U.S. C1 3l0/8.1, 318/116, 318/118,

310/26 Int. Cl l-l0lv 17/00 Field of Search 310/81, 26; 318/116, 118

References Cited UNITED STATES PATENTS 2/l 50 Kreithen 318/118 UX 2,752,512 6/1956 Sarratt 318/116 2,799,787 7/1957 Guttner 318/116 3,434,074 3/1969 Libby A. 310/81 X 3,489,930 1/1970 Shoh 310/81 3,668,486 6/1972 Silver 310/81 x Primary Examiner.l. D. Miller Assistant Examiner-Mark O. Budd Attorney, Agent, or FirmFrank R. Trifari; Bernard Franzblau I 57 I ABSTRACT A control circuit for a transducer comprising a tunable oscillator for driving the transducer at resonance, and a feedback loop responsive to the transducer comprising a phase detector which develops a series of pulse width modulated binary pulses, a DC. source, an integrator, and various switching means to control the tunable oscillator.

8 Claims, 2 Drawing Figures PATENTED JUN25 I974 SHEEI 2 [If '2 ARRANGEMENT FOR GENERATING ULTRASONIC OSCILLATIONS The invention relates to a control circuit for a transducer having a natural resonant frequency, comprising a tunable oscillator having an output circuit coupled to said transducer for applying to said transducer an alternating voltage and an alternating current to vibrate said transducer at its natural resonant frequency, said voltage and said current having a sine phase difference, control switching means having an operative and a nonoperative state, being connected to said oscillator for controlling the duration of the cycle of operation, phase detecting means coupled to said output circuit for detecting the variations in said phase difference and for developing an output signal in response to said variations.

The arrangement according to the invention may be utilized for transducers of the piezomagnetic and the piezoelectric type which are operated either in parallel resonance or in series resonance.

1n known arrangements of this kind transducers having a high quality factor of, for example, 150 1,000 are used to obtain a short operation period. However, known arrangements provided with a transducer having a high quality factor have the drawback that the ultrasonic operations to be performed are not very reproducible, especially in case of intermittent operation in connection with bulk manufacture, and that the arrangement is only usable to a limited extent, for example, exclusively for a limited number of types of ultrasonic welding.

It is an object of the invention to provide an arrangement of the kind described in the preamble in which the reproducibility is considerably extended while maintaining a short operation time of the ultrasonic operations to be performed and which may also be rendered suitable in a simple manner for performing a large number of widely divergent kinds of operations such as: welding, drilling, cleaning.

According to the invention said phase detecting means is arranged to produce an output signal in the form of a series of width modulated binary pulses, said width varying in dependance upon said variations in phase difference; the control circuit further comprises a dc. supply source, integrator means, means for applying the d.c. output of said source to said integrator means to produce an integrator output voltage, means for applying said integrator output voltage to said tunable oscillator, and means including a first and a second switch coupled to said integrator means for effecting the integrator output voltage, means connecting said first switch to said control switching means to adjust the integrator output voltage at a fixed value in the nonoperative state of said control switching means, and means for applying the output signal of said phase detector to said second switch whereby in the nonoperative state of the control switching means the oscillator is tuned at a fixed frequency different from said natural resonant frequency and in the operative state of the control switching means the oscillator is tuned at said natural resonant frequency of said transducer.

The invention will now be described in detail with reference to the accompanying drawing, in which F 16. 1 shows an arrangement according to the invention, while F l0. 2 shows a modification of the arrangement shown in FIG. 1.

1n the arrangement shown according to the invention the ultrasonic energy for exciting a transducer 1 at its natural resonant frequency f of, for example, 20 kHz is derived from an amplifier 2 which is fed by an oscillator 3 at a frequency which is substantially equal to said natural resonant frequency f said transducer 1 for this purpose being connected to the output terminals 4, 5 of the amplifier 2.

The transducer 1 is of the piezo-electric type and is constituted by two rings la and lb of piezoelectric material on which a constant pressure is exerted by means of two end pieces 1d and l f. For this purpose these end pieces are secured to a central bolt lg. For exciting this transducer the rings la and 1b are separated from each other and from the end pieces 1d and If by thin ringshaped soft copper electrodes 1h, 1k, 1m. For the purpose of support and for amplitude transformation a, support In, coupled to ground, and an amplitude transformer 1p are connected to the end piece 1 f.

It is achieved by the given construction of the transducer, particularly by the symmetrical embodiment of the end pieces, the piezo-electric rings and the central bolt, that the said natural resonant frequency of the transducer determines a longitudinal oscillation state (mode), while a transducer is obtained which has a high quality factor of, for example, 500 1,000.

In order to have a correct adaptation of the oscillator frequency to said natural resonant frequency of the transducer in case of load variations occurring, the arrangement is provided with an automatic frequency correction (AFC) loop comprising a phase detector 6 whose output circuit is coupled to a frequencydeterrnining member 7 of the oscillator 3, while a signal dependent on the voltage between the output terminals 4, 5 is applied to this phase detector on the one hand and a signal dependent on the current applied to the transducer is applied on the other hand for automatic correction of the oscillator frequency. In the given embodiment the signal dependent on the voltage between the output terminals 4, 5 is derived after phase shifting by means of a phase shifting network 11 from a central tap of a potential divider constituted by two resistors 8, 9 and the signal dependent on the current applied to the transducer is derived from a resistor 10.

For switching the oscillator 3 on and off, the arrangement is furthermore provided with an electronically formed start-stop device 12 which in this embodiment is'shown as a switch 13 for the sake of simplicity and is provided with two contact terminals 14, 15 and which is connected to the positive terminal of a directvoltage source 16, the contact terminal 14 of said switch being coupled to the direct current circuit of the oscillator 3. The oscillator is switched on by means of this switch by connecting the contact terminal 14 to the positive terminal of the voltage source and it is switched off by connecting said positive terminal to the contact terminal 15 of the switch.

In the arrangement described so far the oscillator frequency follows the natural resonant frequency of the transducer which in turn varies as a result of the variation of a load. Due to this load variation the mutual phase difference between oscillator output current and voltage varies so that a corresponding variation of the output voltage of the phase detector is realized, which varies the oscillator frequency in such a manner that the variation of said mutual phase difference is counteracted. In order to adjust this phase difference at a favourable value the arrangement shown includes the phase shifting network 11 which introduces a suitable phase shift between the two input signals from the phase detector. When the phase shift is adjusted in such a manner (for example, at rr/2) that a phase difference of zero degrees is obtained between oscillator output current and output voltage, transducer 1 is exactly excited at its natural resonant frequency.

In spite of the operation of the AFC-loop it is found that, particularly in the case of intermittent use of the arrangement, the operation of the arrangement does not satisfy the expectations when a transducer having a high quality factor of, for example, 300 1,000 is used. More particularly it is found that the reproducibility of the ultrasonic operations has decreased to a great extent.

In order to realize an accurate reproducibility of all ultrasonic operations under all circumstances with such an arrangement while maintaining a short operation period of, for example, 50 msec. for ultrasonic welding, the output circuit of the phase detector 6 is connected,

according to the invention, to a frequency follower arrangement (18, 19, 20, 21) provided with an integrator 18 which in this embodiment has the form of a capacitor connected to the frequency-determining element 7 of the oscillator 3. The integrator 18 is connected to a first and a second control circuit 19 and 21 which apply oppositely directed control currents. A switch 20 is coupled to the start-stop device 12 being connected to the first control circuit 19 said switch adjusting the integrator 18 at a given voltage value every time after the oscillator 3 is switched off, and by which voltage value the oscillator 3 is tuned through the frequencydetermining element 7 at a defined rest frequency. The second control circuit 21 includes an intermittently operating switching device 33 which is controlled by a pulsatory signal generated by the phase detector 6 which to this end is formed as a pulse duration modulator.

In the embodiment shown the first control circuit is constituted by a current source 19 while the switch 20, likewise as the capacitor 18 operating as an integrator, is provided between the output terminals 22, 23 of the current source 19. More particularly the current source 19 in this embodiment is constituted by a transistor 24, for example, of the pnp-type whose emitter is connected through an emitter resistor 25 and whose base is connected through a Zener diode 26 to the positive terminal of a direct voltage source 27. In addition the base of this transistor 24 is connected through a resistor 28 to a terminal having a fixed reference potential, in this embodiment to ground potential. The output terminals 22, 23 of the current source 19 are connected to the collector of the transistor 24 and to the terminal of fixed reference potential (ground potential), respectively. In this current source the Zener diode 26 operates as a stabilizing element, so that this current source applies a stabilized direct current to the capacitor 18.

The switch 20, provided between the output terminals 22, 23, is constituted by a transistor 29 of the npntype whose emitter circuit is connected to the output terminal 23 and whose collector circuit is connected to the output terminal 22, while for limitation of the current through transistor 29 a collector resistor 30 is included in its collector circuit. For changing over this switch the base of the transistor 29 is connected to a central tap of a potential divider constituted by a cascade arrangement of resistors 31 and 32, which cascade arrangement is provided between the contact terminal 15 of the switch 13 included in the start-stop device and a point of fixed potential (ground potential).

In this embodiment the second control circuit 21 is constituted by a transistor 33 of the npn-type whose collector is connected to the output terminal 22 of the current source 19 and whose emitter is connected through an emitter resistor 34 to the negative terminal of the direct voltage source 27. The transistor 33 is intermittently operative because its base is connected to the output of the phase detector 6, constituted as a pulse duration modulator. As is known this phase detector may be formed, for example, by two Schmitt triggers one of which provides a pulse series I as a function of the value of the current through resistor 10 and the other provides a pulse series V as a function of the voltage across resistor 9. These pulse series are subsequently applied to a gating circuit from which exclusively an output pulse is derived when, for example, a pulse is present in the pulse series I and a pulse is absent in the pulse series V, which pulses are directly applied to the base of the transistor 33.

A pulse series which controls the value of the current through the transistor 33 and whose pulse duration is determined by the phase angle between oscillator output current and output voltage is derived from the output of the pulse duration modulator 6. When, more particularly, this current and voltage are in phase, the pulse duration modulator 6 provides, for example, a pulse signal having a period of T at a pulse duration of T/2 when using the phase-shifting network 11 with a phase shift of and with a period 2T of the oscillator oscillation, so that a distinction is made between a positive and a negative phase angle between oscillator output current and output voltage by using the 90 phaseshifting network 11. Particularly the pulse duration of the output pulses from pulse duration modulator 6 increases, for example, if the oscillator frequency exceeds the natural resonant frequency of the transducer and this pulse duration decreases in the reverse case, but particularly it is achieved with the embodiment of the phase detector 6 as a pulse duration modulator that a variation of the phase angle between oscillator output current and output voltage becomes quickly manifest in its output signal.

By using the switch 20, the oscillator 3 is adjusted at a defined rest frequency every time after it is switched off. In fact, every time after the oscillator 3 is switched off, the transistor 29 becomes conducting so that the capacitor 18 is discharged to a given constant voltage resulting in a rest frequency of the transducer being defined below its natural resonant frequency.

When switching on the oscillator 3, the current through transistor 29 is interrupted and the direct current from the current source 19 is applied to the capacitor 18. Since the oscillator frequency at this instant of switching on is located below the said natural resonant frequency, the current through transistor 33 is mainly interrupted by the output pulses from the pulse duration modulator 6 during a period of the oscillator oscillation so that the capacitor voltage and hence the oscillator frequency and consequently the duration of the output pulses from pulse duration modulator 6 increase until for a pulse duration of T/ 2 a balanced state is established in which with an average per unit of time as much charge is applied to the capacitor 18 by current source 19 as is derived therefrom by the control circuit 21.

By using the steps according to the invention the oscillator is tuned and stabilized at the said natural resonant frequency under all circumstances; every time after switching off, the oscillator 3 is tuned through switch 20 at a fixed defined rest frequency located below the said natural resonant frequency and after it is switched on it is detuned into the direction of and tuned at the said natural resonant frequency of the transducer.

Not only is a defined rest frequency of the oscillator realized with the arrangement according to the invention, but also a very rapid control of the oscillator frequency to the natural resonant frequency is achieved. In fact a variation of the phase angle between oscillator output current and output voltage becomes immediately manifest in a variation of the duration of the output pulses from the pulse duration modulator on the one hand and on the other hand the time constants for charging and discharging the capacitor 18 are very short. In fact, as viewed from the capacitor, this charging and discharging is ensured by current sources which per unit of time apply or deplete a charge current independent of the instantaneous capacitor voltage to or from the capacitor so thatalso the speed of detuning the oscillator is independent of this instantaneous capacitor voltage.

Particularly when using a transducer having a high quality factor, for example, in the order of 300 1,000 the steps according to the invention lead to a very good reproducibility of the operations to be performed. On the one hand it is always ensured that the oscillator, after being switched on, is not stabilized at one of the many parasitic resonant frequencies of the transducer and on the other hand the rapid operation of the frequency follower arrangement and the pulse duration modulator causes an abrupt detuning of the oscillator frequency in case of an abrupt variation of the load and hence of the natural resonant frequency of the transducer so that oscillator and transducer are again tuned to the natural resonant frequency, for example, within periods of the oscillator oscillation. In addition the rapid control of the oscillator frequency realizes an accurate tuning of the oscillator at the natural frequency. This accuracy is still further increased by connecting, as is shown in the embodiment, the emitter of the transistor 33 through the emitter resistor 34 to the negative terminal of the voltage source 27. It is achieved thereby that the control circuit 21 derives a current of constant value which is smaller than the charge current from the capacitor 18 even in the absence of output pulses from pulse duration modulator 6 so that a still better defined discharge current is obtained. Particularly it is found that the phase error emanating from this control amounts to less than one degree.

The steps according to the invention not only lead to an accurate reproducibility of the ultrasonic operations, but a particularly flexible and universal arrangement is obtained because this reproducibility of the operations is maintained for all possible values of the load as well as for all possible load variations. In addition it is achieved that for each kind of operation as well as for each kind of material to be worked the value of the required amplitude can be adjusted in a simple manner, namely by forming the resistor 34 in the control circuit 21 as an adjustable resistor. In fact, the phase angle between oscillator output current and output voltage is determined by the values of the current derived from capacitor 18 by circuit 21 so that the oscillator and hence the transducer can be operated at any suitable frequency within the resonance curve of the transducer, which frequencies within the said resonance curve each characterize a given amplitude of the mechanical oscillation of the transducer.

FIG. 2 shows a modification of the frequency follower arrangement which largely corresponds to that of FIG. 1. Also in this embodiment the arrangement is provided with an integrator 18 to which a first and a second control circuit are connected, while a switch 35 coupled to the start-stop device is connected to the first control circuit and the second control circuit includes the intermittently operating switching device 33.

More particularly the integrator is constituted by a so-called Miller integrator consisting of an operational amplifier 37 and the conventional series resistor 38, which amplifer is shunted by a capacitor 39. To adjust the capacitor voltage the control current in the first control circuit flows from the positive terminal of the direct voltage source 27 through resistors 40 and 38, capacitor 39 and the operational amplifier 37 to the negative terminal of the voltage source 27, while in the second control circuit the control current flows from the emitter of transistor 33 through the operational amplifier 37, the capacitor 39 and the resistor 38 to the collector of transistor 33.'

- Likewise as in the embodiment of FIG. 1 the base of transistor 33 is controlled by the duration-modulated output pulses from the phase detector 6. The absence of such an output pulse is in this case an output voltage of zero volts and the presence is a positive output voltage. These output pulses are applied through an inverter 41, a NOR-gate 42 and a resistor 43 to the base of transistor 33. For dc-biassing the base of this transistor is also connected through a resistor 44 to the negative terminal of source 27.

Also in this embodiment the oscillator is tuned to a defined rest frequency every time after it is switched off. To this end the terminal 15 of the start-stop device is connected at one end to the NOR-gate 42 and at the other end to the base of the switch 35 formed as a transistor. Transistor 35 together with an emitter resistor 45, is connected in parallel with the operational amplifier 37 at one end and at the other end through a second switch, likewise constituted by a transistor 46 provided with an emitter resistor 47, to the positive terminal of the 'direct voltage source 27. Likewise as the base of transistor 35, the base of transistor 46 is coupled to the terminal15 of the start-stop device.

It is to be noted that by the rapid operation of both the pulse duration modulator and the frequency follower arrangement the proportioning of the resistor 30 of FIG. 1 or resistor 45 of FIG. 2 can be chosen to be such that the rest frequency of the oscillator is above and in the immediate vicinity of the natural resonant frequency. The arrangement may alternatively be formed with its electrical dual elements; for example, an inductor may be used instead of a capacitor in the integrator. I

For the arrangement shown in FIG. 1 the following data are mentioned:

Transistor 24 BCY Transistor 29 2 N930 Transistor 33 2 N930 Capacitor 18 Mepolesco of 5.6 pF

Zener diode 26 BZY 88*C3V3 Resistor 2.7 K D Resistor 28 l8 k D Resistor 30 200 Q Resistor 31 100 K S) Resistor 32 22 K .(2

Resistor 34 10 K Q.

What is claimed is:

l. A control circuit for a transducer having a natural resonant frequency, comprising a tunable oscillator having an output circuit coupled to said transducer for applying to said transducer an alternating voltage and an alternating current to vibrate said transducer at its natural resonant frequency, control switching means having an operative and a non-operative state, means connecting said control switching means to said oscillator for controlling the duration of the oscillator cycle of operation, phase detecting means coupled to said output circuit for detecting a variation in phase difference between said alternating voltage and current and for developing an output signal comprising a series of pulse width modulated binary pulses in response to said phase variation, said pulse width varying in dependance upon said variations in phase difference, a dc supply source, integrator means, means for applying the dc. output current of said source to said integrator means to produce an integrator output voltage, means for applying said integrator output voltage to said tunable oscillator to control the frequency thereof, first and second switches coupled to said integrator means for controlling the integrator output voltage, means connecting said first switch to said control switching means so as to adjust the integrator output voltage at a fixed value in the non-operative state of said control switching means whereby the oscillator is tuned to a fixed frequency different from said natural resonant frequency, and means for applying the output signal of said phase detector to said second switching whereby in the operative state of the control switching means the oscillator is tuned via the second switch and the integrator means to said natural resonant frequency of said transducer.

2. A control circuit as claimed in claim 1, characterized in that said fixed frequency at which the oscillator is tuned in the non-operative state of said control switching means is lower than the natural resonant frequency of the transducer.

3. A control circuit as claimed in claim 1, characterized in that said do supply source is comprises by a direct current source and the integrator means comprise a capacitor.

4. A control circuit as claimed in claim 1, wherein said second switch comprises a transistor having an emitter resistor in its emitter circuit, the base of said transistor being connected to the output circuit of said phase detector.

5. A control circuit as claimed in claim 4, characterized in that the resistance of said emitter resistor is adjustable.

6. Apparatus for controlling the frequency of a transducer having a natural resonant frequency comprising, a tunable oscillator having an output circuit coupled to said transducer for applying to said transducer an alternating voltage and an alternating current to operate said transducer at its natural resonant frequency, control switching means having first and second states and coupled to said oscillator for controlling the on-off oscillations thereof, phase detecting means coupled to said oscillator output circuit and responsive to a phase variation between said alternating voltage and current to develop an output signal comprising pulse width modulated pulses, the pulse width being determined by the phase angle between said alternating voltage and current, integrator means coupled to said tunable oscillator to control the oscillator frequency, a source of electric energy coupled to said integrator means to derive an integrator output signal, a first switching device controlled by said control switching means and coupled to said integrator means to control the integrator output signal so that in the first state of said control switching means the oscillator is tuned to a frequency different than said natural resonant frequency, and a second switching device controlled by the output signal of the phase detecting means and coupled to said integrator means to control the integrator output signal so that in the second state of said control switching means the oscillator is tuned to said transducer natural resonant frequency.

7. Apparatus as claimed in claim 6 wherein said integrator means comprises a capacitor, and said first switch comprises a semiconductor control element connected in shunt with said capacitor and with said electric energy source.

8. Apparatus as claimed in claim 7 wherein said energy source comprises a DC source of constant current and said first and second switching devices are connected to control the capacitor charge and discharge independently of the value of the capacitor voltage, and said phase detecting means responds to a change in the phase angle between the alternating voltage and current so as to alter the capacitor voltage and hence the oscillator frequency in a direction to counteract said phase angle change.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3 3 19 9 1 DATED June 25, 1974 |NV ENTOR(S) I IVAN M. G. P, BOURGEOIS ET AL It is certified that er ror appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 3, line 2, cancel "is"; cancel "by";

line 3, change "comprise" to comprises Signed and Sealed this A ttes t:

RUTH C. MASON c. M-ARSHALL DANN .4 [testing Officer (nmmixsl'uncr ofPaIenls and Trademarks UNITED sTATEs PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,819,961 DATED June 25, 1974 V IVAN M. G, P, BOURGEOIS ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 3, line 2, cancel "is"; cancel "by";

line 3, change "comprise" to comprises Signed and Sealed this Fourteenth Day Of September 1976 [SEAL] Attest.

RUTH C. MASON C. M ARSHALL DANN Arresting Officer (ummissimu'r nj'Parenrs and Trademarks UNITED sTATEs PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,819,961 DATED June 25, 1974 V IVAN M. G, P, BOURGEOIS ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 3, line 2, cancel "is"; cancel "by";

line 3, change "comprise" to comprises Signed and Sealed this Fourteenth Day Of September 1976 [SEAL] Attest.

RUTH C. MASON C. M ARSHALL DANN Arresting Officer (ummissimu'r nj'Parenrs and Trademarks 

1. A control circuit for a transducer having a natural resonant frequency, comprising a tunable oscillator having an output circuit coupled to said transducer for applying to said transducer an alternating voltage and an alternating current to vibrate said transducer at its natural resonant frequency, control switching means having an operative and a non-operative state, means connecting said control switching means to said oscillator for controlling the duration of the oscillator cycle of operation, phase detecting means coupled to said output circuit for detecting a variation in phase difference between said alternating voltage and current and for developing an output signal comprising a series of pulse width modulated binary pulses in response to said phase variation, said pulse width varying in dependance upon said variations in phase difference, a d.c. supply source, integrator means, means for applying the d.c. output current of said source to said integrator means to produce an integrator output voltage, means for applying said integrator output voltage to said tunable oscillator to control the frequency thereof, first and second switches coupled to said integrator means for controlling the integrator output volTage, means connecting said first switch to said control switching means so as to adjust the integrator output voltage at a fixed value in the non-operative state of said control switching means whereby the oscillator is tuned to a fixed frequency different from said natural resonant frequency, and means for applying the output signal of said phase detector to said second switching whereby in the operative state of the control switching means the oscillator is tuned via the second switch and the integrator means to said natural resonant frequency of said transducer.
 2. A control circuit as claimed in claim 1, characterized in that said fixed frequency at which the oscillator is tuned in the non-operative state of said control switching means is lower than the natural resonant frequency of the transducer.
 3. A control circuit as claimed in claim 1, characterized in that said d.c. supply source is comprises by a direct current source and the integrator means comprise a capacitor.
 4. A control circuit as claimed in claim 1, wherein said second switch comprises a transistor having an emitter resistor in its emitter circuit, the base of said transistor being connected to the output circuit of said phase detector.
 5. A control circuit as claimed in claim 4, characterized in that the resistance of said emitter resistor is adjustable.
 6. Apparatus for controlling the frequency of a transducer having a natural resonant frequency comprising, a tunable oscillator having an output circuit coupled to said transducer for applying to said transducer an alternating voltage and an alternating current to operate said transducer at its natural resonant frequency, control switching means having first and second states and coupled to said oscillator for controlling the on-off oscillations thereof, phase detecting means coupled to said oscillator output circuit and responsive to a phase variation between said alternating voltage and current to develop an output signal comprising pulse width modulated pulses, the pulse width being determined by the phase angle between said alternating voltage and current, integrator means coupled to said tunable oscillator to control the oscillator frequency, a source of electric energy coupled to said integrator means to derive an integrator output signal, a first switching device controlled by said control switching means and coupled to said integrator means to control the integrator output signal so that in the first state of said control switching means the oscillator is tuned to a frequency different than said natural resonant frequency, and a second switching device controlled by the output signal of the phase detecting means and coupled to said integrator means to control the integrator output signal so that in the second state of said control switching means the oscillator is tuned to said transducer natural resonant frequency.
 7. Apparatus as claimed in claim 6 wherein said integrator means comprises a capacitor, and said first switch comprises a semiconductor control element connected in shunt with said capacitor and with said electric energy source.
 8. Apparatus as claimed in claim 7 wherein said energy source comprises a DC source of constant current and said first and second switching devices are connected to control the capacitor charge and discharge independently of the value of the capacitor voltage, and said phase detecting means responds to a change in the phase angle between the alternating voltage and current so as to alter the capacitor voltage and hence the oscillator frequency in a direction to counteract said phase angle change. 